When can I tell whether they are male or female?
It only becomes possible to tell whether cannabis plants are male or female once the flowering cycle has begun.
Cannabis begins to flower when it receives 12 hours of light and 12 hours of uninterrupted darkness in each 24-hour cycle. Once this light cycle (or photoperiod) is initiated, plants enter the pre-flowering stage, when they increase in size as the branch structure prepares to bear and support flowers. During pre-flowering, which usually lasts about 10 days, it is still not possible to identify the sex of the plant. The first flowers should be visible within two weeks.
Female cannabis plant:
Male cannabis plant:
Inside, it is the grower who decides when to alter the photoperiod and induce flowering. This is achieved by simply adjusting the timers on the lights.
Outside, the grower must wait until the natural daylight hours have decreased to approximately 12 per day. Exactly when this occurs will of course vary depending on the locality. In Northern Europe it happens around mid-to-late July.
It is possible to induce flowering outside by allowing the plants 12 hours of daylight each day, then covering them or taking them inside. This must however be done every day without fail. If the periods of darkness are interrupted by light, this can adversely affect the plants, setting it back to a vegetative state.